The present invention relates to an automatic machine for grinding and bevelling glasses of spectacles.
Conventional machines for grinding and bevelling ophthalmic glasses comprise a U-shaped carriage which is slidably and pivotally mounted on a fixed horizontal shaft, a second horizontal shaft rotatively mounted in a perpendicular manner between the branches of the U-shaped carriage and consisting of two parts between which the glass to be machined is gripped, and a third rotatable horizontal shaft carrying at least one grinding wheel, and more particularly edge-trimming and bevelling grinding wheels.
When an optical glass blank having a thick edge surface has been cut to the shape of the frame, the edge surface of the glass has a cylindrical shape and a bevelling operation must be carried out by guiding the glass so as to form a V-sectioned bevel on the edge surface of the glass which must be located between the edges of its periphery, said projecting bevel being adapted to engage in the groove of the rim of the frame.
Machining such a bevel with manual control means is extremely difficult.
It has already been proposed to employ a "free bevel" system in which the edge surface of the blank is brought into a V-sectioned groove of a grinding wheel and the shaft carrying the glass is left free to move in translation during the grinding operation.
However, this system does not give a fully satisfactory result.
It is indeed desirable to construct a device which
an permits obtaining a guided bevel on the edge surface of ophthalmic glass automatically and with high precision.
Document EP-A-0281,480 proposed for this purpose a machine for grinding and bevelling ophthalmic glasses of the aforementioned type which comprises a follower adapted to remain in contact with the edge surface of a shaped blank, in a position perpendicular to said edge surface, and adapted to measure the distance L1 from the edge of the front face of the glass relative to that of a flat glass in which L1=0.
This solution is not fully satisfactory, since the construction of the sensor, which is a mechanical sensor of complex and costly structure, is difficult to achieve. Further, the follower is of fragile construction and the contact of the latter with the edge surface of the glass results in a wear of the follower; moreover, the precision of the data obtained is too closely dependent on the construction of the follower.
It has already been proposed in document DE-A-3,842,601 to use a detecting fork each of the two teeth of which extends on one of the two sides of the blank grinding wheel and against which the front convex face and rear concave face of the glass to be ground abut.
The arrangement described in this document is particularly complex and costly and the two branches of the detecting fork, which must be resiliently deformable, are particularly fragile.